Storage and transportation of aluminium strip

ABSTRACT

The invention is particularly directed to the problem of creep deformation in a coiled aluminum strip, which occurs during coiling and for a period thereafter. The problem arises because the profile of the strip across its width is not flat, and is in fact usually thicker in the middle than at the edges (positive crown). To compensate for this, the invention provides that the spool onto which the coil is wound is adapted to provide more support to the strip in the center than at the edges, by making the central portion of the spool of a greater diameter than the end portions of the spool. A strip having a positive crown which is wound onto such a spool was found to exhibit significantly reduced creep strain, leading to reduced creep deformation.

FIELD OF THE INVENTION Background Of The Invention

The present invention relates to a spool suitable for use in the storageand transportation of strip material made of aluminium or an alloythereof, and to a method of coiling such material on a spool.

SUMMARY OF THE INVENTION

Aluminium strip material, such as that used in lithographic printing, iscoiled under tension on large steel or fibre spools for storage andtransportation. The spool is a large cylinder that has a uniform outerdiameter and a length sufficient to completely support the width of thestrip material, often in practice extending beyond the strip for a shortdistance either side. It is known that the coiling of aluminium stripmaterial can affect the flatness of the strip. Aluminium strip materialthat was flat immediately before it was coiled onto a spool can becomeoff-flat as the strip creeps under the uneven stresses that arise acrossthe width of the strip. Aluminium presents a particular problem incoiling because it is much more prone to creep than, for example, steel.

The non-uniform stresses across the width of the strip when it is coiledarise from the fact that the thickness of the strip varies slightlyacross the width of the strip, with the strip usually being slightlythicker in the middle than at the edges (a positive crown). Thisvariation in thickness results in the coil being slightly barrel-shaped,i.e. the coil has a larger diameter at its middle than at its edges.This further results in the middle of the coil carrying more of thecoiling tension than the edges.

The manufacturing process for aluminium strip generally tries to ensurethat the strip does have a positive crown since strip with a negativecrown (implying that the outer edges are thicker than the centre) canresult in unpredictable handling, particularly during later fabricationprocesses. Because the manufacturing process is a multi-step process, amargin of error needs to be built in to ensure that no part of theoutput has a negative crown. Thus the manufacturing process is set todeliberately provide a crown, typically such that the thickness in thecentral section is at least about 0.3% higher than that at the twoopposing edge sections. Bearing the margin of error in mind, thisgenerally ensures that, at no point in the strip, is the crown such thatthe central section is less than about 0.1% greater in thickness thanthe opposing edge sections. Typically however the manufacturing processis set so that the crown is such that the central section isapproximately 0.5% greater in thickness than the opposing edge sections,but up to 1% or even higher is possible, with 2% the practicablemaximum.

Creep occurs during coiling, when it may be made easier by the slightwarming of the aluminium that often occurs during cold rolling or duringpre-treatment processes such as cleaning or during stoving afterpainting. Creep continues in the coil even at room temperature, untilthe stress is relaxed to the extent that the creep rate becomesinsignificant.

As each lap of the aluminium strip is coiled under tension about thespool, each new lap imposes an incremental inward pressure on thematerial that has already been coiled onto the spool. This results inthe flatness of the strip varying with respect to its position in thecoil. For example, the strip from the outer laps of the coil (otherwisereferred to as wraps) can buckle along the centre line of the stripwhilst strip from the inner wraps can buckle along its edges. The formerdeviation from flatness is termed ‘long-middle’ whereas the latterdeviation from flatness is termed ‘wavy edges’.

When the aluminium strip is being coiled onto the spool, the spool ismounted on a mandrel which rotates the spool during the coilingprocedure. Once the coiling of the strip has been completed, the spoolis removed from the mandrel. Unfortunately, especially with fibrespools, the spool can deform under the pressure from the coiled stripwhich can further exacerbate the problems mentioned above with respectto off-flatness. The compressive force from the coil causes the spool toradially displace inwards which makes the inner laps shorter and socauses the tension in the inner laps to be reversed. FIG. 1 is a modelprediction of creep strain (in i units) for a coil on a conventionalspool 24 hours after coiling. The compressive (−ve) strain for the innerlaps at the middle of the strip can be clearly seen along with a largepositive strain either side of the middle region of the strip. Thismodel thus predicts the strip at the inner laps to have wavy edges withquarter pockets. Quarter pockets are formed by the buckling of the stripalong parallel longitudinal lines inboard from each of the longitudinaledges of the strip approximately a distance equal to a quarter of thetotal width of the strip.

Schnell et al (Metallwissenschaftund Technik vol. 8 August 1986) havedescribed the problem of off flatness and attempted to explain theseeffects but have not proposed any solution.

Attempts have been made to reduce the off-flatness caused by coiling butthese attempts have generally focused on post processing of the strip tostraighten the strip. However, in JP11-179422 a method is described forcontrolling the flatness of steel strip material that has a convex crownwhich utilises a contoured spool having a concave crown.

JP 09-057344 and JP 09-076012 both describe similar methods of windingsteel strip material onto a mandrel. In both cases a narrow sleevedefining a convex crown is fitted on the mandrel and is positionedcentrally of the width of the steel strip being coiled.

The present invention seeks to provide a system and a method of coilingaluminium strip on a spool in such a way as to reduce the deformation ofthe strip resulting from creep, and thereby improve the flatness of thestrip. The present invention is particularly concerned with reducing thewavy-edge off-flatness in the inner laps of a coil of aluminium stripmaterial.

As already mentioned, a conventional cylindrical spool defines an outersupporting surface for the strip material which is cylindrical in shape.If the strip material were of a constant thickness across its width,then the spool would provide a substantially constant support across thewidth of the strip material and the uneven stresses which cause creepwould not arise. However, where the strip has a positive crown, theconventional spool gives a greater support to the strip at its middlethan at its edges, the exact profile of this variation depending on theshape of the profile across the strip. The aforementioned JP11-179422seeks to cater for this by providing that the external shape of thespool inversely matches the external shape of the strip across itswidth, the purpose being to try to negate the uneven stresses caused bythe variation in the thickness of the strip across its width, to thusemulate the situation which would occur if the strip had a constantthickness across its width; hence, for a strip having a positive crownthe external shape of the spool is concave, and vice-versa.

In a first aspect of the invention there is provided a system forcoiling of aluminium strip material, said system consisting of a coilassembly comprising a mandrel, a spool removably mounted on said mandreland an aluminium strip material having a positive crown, said coilassembly having a supporting surface on which is to be coiled said stripmaterial, and wherein the coil assembly is adapted so that itssupporting surface provides a support profile in which the supportprovided by that part of the supporting surface which supports the crownis greater than that provided by the remaining part or parts of thesupporting surface during coiling of at least the inner laps of thestrip material.

The normal natural consequence of the rolling process by which the stripmaterial is made is that the crown is positioned approximately centrallywith respect to the width of the strip material; however, subsequentprocessing, for example the slitting of a wider strip to form narrowerones, may result in the crown being off-centre when it is coiled. Theteaching of the present invention can be applied whatever the positionof the crown, but it will be assumed herein that the crown isapproximately centrally positioned with respect to the strip material,in which case, the support provided to the central portion of the stripmaterial will be greater than that provided to opposing edge portions ofthe strip material during coiling of at least the inner laps of thestrip material.

The support profile of the supporting surface may be provided byadaption of the shape and/or properties of the spool, or by adaption ofthe strip material to be wound thereon, or a combination of both.

Adaption of the coil assembly to enable its supporting surface toprovide the required support profile may be achieved in a number ofways. For example, the spool may be contoured to define a supportingsurface which has a diameter at a central region which is greater thanat its end regions. Thus, during coiling of the strip material, a largertensile stress is applied to the central region of the strip than to itsend regions, particularly in the inner laps of the coil.

The interface between the greater diameter in the central region and thelesser diameter at the end regions may be by way of one or more steps,or may be a smooth transition, or a combination of both, according tothe circumstances. Thus the contour of the supporting surface may varyfrom a smooth convex surface, extending across the expected width of thestrip material to be coiled, to a stepped cylindrical surface in whichthe central region has a greater diameter than the end regions, thecentral region having a width less than the width of the strip materialto be coiled.

The use of a spool having such a convex supporting surface acts to alterthe distribution of stress in the inner laps of the coiled strip,thereby reducing subsequent creep strain. Using the contoured spool ofthe present invention, the concentration of coiling tension in themiddle of the strip width arises at the start of coiling. This reducesthe amount of strip that must be discarded from the inner laps of a coilwhere strict flatness requirements apply. In contrast, on a normal plaincylindrical spool, the concentration of coiling tension arises onlyafter some laps have been coiled. Hence, the present invention is ofparticular benefit when used with aluminium strip materials for whichthere are strict flatness requirements such as materials used inlithographic printing.

The required support profile may be achieved by altering the externalphysical profile of the spool itself, or by adding profiling elements toan otherwise plain cylindrical spool, or a combination of bothtechniques may be used. Thus, for example, a profiling element in theform of a sleeve may be fitted over the central region of a plaincylindrical spool to increase the effective diameter of the supportingsurface of the spool in its central region. Such a sleeve will have alength which is less than the width of the strip material to be coiled.This arrangement has the advantage that a plain cylindrical spool can beused; such spools can be manufactured very cheaply by simply cutting offsuitable lengths from an elongate tube. Anything more complicated, suchas a profiled tube, is likely to have to be manufactured as anindividual item and is thus much more costly. In the industry, spoolsare regarded as throw-away items and therefore cost is an importantfactor.

Another way of utilising a plain cylindrical spool is to realise theaforementioned profiling element as the leading end of the stripmaterial itself, for example by providing that the strip is formed, atits leading end, with a tongue which is narrower in width than theremainder of the strip The tongue has a length, in the longitudinaldirection of the strip, which is approximately equal to thecircumference of the outer surface of the spool. Thus, as coilingcommences, the first lap is formed by the narrow tongue which thuseffectively forms a profiling element as described above. The thicknessof the tongue, and hence the profiling element so formed, isconveniently equal to the thickness of the strip material; if athickness greater than this is required, then the length of the tonguecan be increased to provide two or even more turns, before the fullwidth of the strip commences. Preferably the length of the tongue isequal to n times the outer circumference of the spool, where n is aninteger.

In an embodiment, the width of the tongue increases from a smaller widthto the full width of the strip material during the first few laps of thestrip material about the coil assembly.

Another way of adapting the aluminium strip material to provide therequired support profile is an arrangement in which a sheet of, forexample, aluminium, is attached, for example by adhesive, mechanicalfixing, welding, or spot welding to a surface of the leading end of thestrip material, said sheet having a width narrower than that of thestrip material, and being centrally located with respect to the width ofthe strip material, said sheet of material being effective, as the stripmaterial is coiled, to provide the spool with an effective outerdiameter at a central region of the spool that is greater than theeffective outer diameter of the spool at opposing end regions of thespool. Preferably said sheet of material has a length, in thelongitudinal direction of the strip material, which is approximatelyequal to n times the outer circumference of the spool, where n is aninteger.

An alternative way of adapting the spool to provide the required supportprofile is to alter the support strength provided by the spool along thelength of its supporting surface. When the strip is coiled onto thespool, compressive forces act radially inwards on the spool, thuscausing compression of the spool material. Conventionally, the spool isconstructed with a constant cross section in the direction of its axis,at least along that part of its length which defines the supportingsurface. This ensures that any distortion of the spool caused by thesecompressive forces is substantially constant across the width of thestrip material being coiled. If, however, the cross section is notconstant along the axis then the effect of the compressive forces willbe different across the length of the supporting surface. Thistranslates into a different effective support for the strip materialbeing coiled according to its position across the width. Thus, forexample, if the cross section of the centre region of the spool isgreater than at the end regions, then the required support profile canbe achieved even if the supporting surface itself has a conventionalplain cylindrical shape. A similar effect can be achieved by weakeningthe support which the material of the spool is capable of providing incertain select regions by removing material to reduce its strengthwithout necessarily changing the shape of the supporting surface itself.For example, the support which the end regions of the supporting surfaceprovides can be reduced with respect to that provided by the centralregion by cutting slits into the material of the spool to form fingersat the ends, which partially collapse (i.e. move inwards) when the coilis wound onto the spool.

A further way of adapting the spool so that its supporting surfaceexhibits the required support profile is to vary the stiffness orrigidity of the material of the spool along its length, for example byforming the central region of a material having a greater stiffness orrigidity than the material of the opposing end regions. This can bechanged by altering the inherent stiffness or rigidity of the materialitself, or by locally weakening the material by forming apertures orslits, somewhat in the manner discussed above.

It has already been mentioned that, in conventional practice, the spoolis mounted on a mandrel, the mandrel being caused to rotate the spoolduring coiling. It is possible to use the mandrel to adapt an otherwiseconventional spool to cause its supporting surface to provide a supportprofile which varies along its length in the manner described above.Thus, for example, the mandrel may be such as to deform the spool whenin place on the mandrel such that the diameter of the supporting surfaceof the spool in the central region is greater than that at the opposingend regions. In such a case, the mandrel would normally be of theexpanding type, whereby it could be collapsed for removal after coilingis completed.

A combination of these various techniques can be used to achieve thedesired support profile.

In an embodiment, the spool is adapted such that the support profile ofits supporting surface matches, at least approximately, the shape of agraph representing the radial displacement of an outer lap of a stripmaterial of the same type as that to be coiled, which strip material hasbeen coiled on a conventional right cylindrical spool, after removal ofthe mandrel.

In a second aspect the present invention provides a method of coilingaluminium strip material having a positive crown wherein the stripmaterial is fed to a coil assembly comprising a spool and a mandrel; thecoil assembly is rotated thereby coiling the strip material about asupporting surface of the coil assembly; and thereafter the mandrel isremoved, said method being characterised in that, during coiling of atleast the inner laps of the coiled strip material, the coil assembly isadapted so that its supporting surface provides a support profile inwhich the support provided by that part of the supporting surface whichsupports the crown is greater than that provided by the remaining partor parts of the supporting surface.

In a further alternative, either alone or in combination with the aboveaspects of the invention, a tension force is applied to the aluminiumstrip as it is being coiled. Tension is not applied until the leadingend of the strip has become firmly gripped to the spool, this usuallybeing shortly after the turns begin to overlap at the completion of thefirst lap. Preferably the initial laps of the strip are coiled at afirst higher tension and a second lower tension is applied to later lapsof the strip as it is being coiled. Thus, most of the coil is coiledwith the strip under a nominal tension, sufficient to hold the coiledcoil in a stable state for storage and transportation. This second(nominal) tension is preferably at least 10% lower than the first,higher, tension and is more preferably at least 20% lower. In addition,the second tension is preferably no greater than 80% lower than thefirst tension and is more preferably no greater than 50% lower. Thecoiling tension may be continuously reduced from the higher tension tothe lower tension and this reduction to the lower tension is preferablyperformed during the first half of the total laps of the coil. This isillustrated conceptually in FIG. 17 which shows a short level section(curve a) at a higher tension, followed by the remainder at a lowertension—the nominal tension. The transformation from the higher tensionto the nominal tension may be relatively rapid, as shown by curve a, ormay be slower, with or without a shorter section at the higher tension,as shown by curves b and c. The tension build-up associated with thefirst lap is not shown.

Reference herein to aluminium is to be understood as a reference toaluminium and its alloys.

Reference is also made herein to flatness and to off-flatness. In thecontext of this document off-flatness is to be understood to be thedifference in strain across the width of the strip as measured atdifferent positions along the longitudinal or coiling direction of thestrip.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample with reference to and as shown in the accompanying drawings, inwhich:

FIG. 1 illustrates a model prediction of the creep strain for analuminium strip coiled on a conventional spool;

FIG. 2 is a schematic perspective view of a spool in accordance with thepresent invention;

Fig. 2A is a schematic perspective of another embodiment of a spool inaccordance with present invention;

FIG. 3 illustrates a model prediction of the radial displacement of theouter lap of an aluminium strip coiled on a conventional rightcylindrical spool after removal of the mandrel;

FIG. 4 illustrates a model prediction of the distribution of hoop stressacross the width of three different positions in a coil during coilingon a conventional spool, and after removal of the mandrel;

FIG. 5 illustrates a model prediction of the distribution of hoop stressacross the width of the same three laps as for FIG. 4 during coiling ona spool, and after removal of the mandrel, in accordance with thepresent invention;

FIG. 6 illustrates a model prediction of the distribution of hoop stressacross the width of the same three laps as for FIG. 4 during coiling onan alternative spool, and after removal of the mandrel, in accordancewith the present invention;

FIG. 7A, B, C are diagrammatic plan views of the leading end of analuminium strip to be coiled, showing shaped end sections;

FIG. 8 is a diagrammatic plan view of the leading end of an aluminiumstrip to be coiled, showing a modified end section.

FIG. 9 illustrates a model prediction of the creep strain across thewidth of the first lap 5 mm radially from the spool immediately aftercoiling for a conventional spool and for a spool in accordance with thepresent invention and a spool similar to the prior art spool ofJP11-179422;

FIG. 10 illustrates a model prediction of the creep strain for analuminium strip coiled on a spool having a centre sleeve in accordancewith the present invention, 24 hours after coiling;

FIG. 11 illustrates a model prediction of creep strain with respect toinitial coiling tension and spool contour immediately after coiling;

FIG. 12 illustrates a model prediction of creep strain with respect toinitial coiling tension and spool contour 24 hours after coiling;

FIGS. 13 to 16 are graphs of position across strip width againstposition along strip length illustrating the results of various testscarried out on coiled strips; and

FIG. 17 is a graph to illustrate the variation of applied coiling stressas the coiling proceeds;

FIG. 17 a is a front view of a mandrel on a spool in accordance with thepresent invention;

FIG. 17 b is a front view of the mandrel of FIG. 17 a with the spoolremoved therefrom;

FIG. 18 is a front view of a sleeve and a spool in accordance with thepresent invention;

FIG. 19 is a front view of a spool having a strip forming a crown woundthereon;

FIG. 20 is a perspective view of a strip of a material wound on a spoolin accordance with the present invention;

FIG. 21 is a top view of spool with the strip of a material with anarrower sheet of material for attachment to the strip material; and

FIG. 22 is an end view of a mandrel, a tensioning roll, and spool havinga strip wound thereon in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMOBODIMENTS

A spool 1 for use in the storage and transportation of aluminium stripmaterial is shown in FIG. 2. The spool 1 is approximately cylindricalbut has a central crown region 2 where the outer diameter of the spoolis greater than at the edge regions 3. The length of the spool is suchas to fully support the strip material, which means in practice that thespool is at least as long as the width of the strip, and may indeed belonger; however, under certain circumstances, the spool may be veryslightly shorter—perhaps by up to about 50 mm—than the width of thestrip to meet certain specialist requirements. The outer diameter of thespool increases continuously to a plateau of uniform diameter from theedge regions 3 to the centre region 2. The difference between thediameter of the end and centre regions can be as great as 10 mm or more.For some applications the edge regions 3 can be cut away to leave only anarrow spool supporting just the centre of the coil. Such a narrow spoolor a spool having a very high crown region 2 could mark the inner lapsof the coil. The preferred difference in height between the edge regions2 and the crown 3 is 0.02 to 1.0 mm, preferably 0.05-0.3 mm still morepreferably 0.05 to 0.10 mm.

As show in FIG. 2A, the spool 1 may have slits 4 extending from eachend, for example, one-quarter of the entire length of the spool. Thespool may also have a substantially uniform diameter.

The shape of the spool 1 may alternatively match the profile shown inFIG. 3 which is a model prediction of the radial displacement of anouter lap on a right cylindrical spool after removal of the mandrel. Ascan be seen, the maximum displacement of the strip, in this case, 0.07mm, is at the centre of the strip and the displacement rapidly decreasesto zero from the maximum over a central region approximately 800 mmwide. However, the maximum displacement will depend on the height of thecrown on the strip and the number of laps in the coil. Where the spool 1has the shape shown in FIG. 3 the distribution of hoop stress whilecoiling the inner laps of the aluminium strip would be similar to thedistribution of hoop stress for the outer laps. The hoop stress is ameasure of the tension force, acting in the circumferential direction ofthe coiled strip, per unit cross section area of strip.

The effect of coiling an aluminium strip on a spool 1 modified inaccordance with the invention is illustrated with reference to FIGS. 4to 6. In FIG. 4 the distribution is shown of hoop stress across thewidth of three laps during coiling on a conventional right cylindricalspool. As can be seen the coiling tension is carried by in excess of themiddle 800 mm of strip width whilst the innermost position is beingcoiled but this is reduced to only 600 mm when coiling the thirdposition. This effect saturates after approximately 50 mm build-up ofcoil. After the mandrel is removed from the spool the reversal of thestress extends over the middle 500 mm of strip width and leaves quarterpockets of residual tension in the strip either side of the largecompressive stress, at the inner position.

In FIG. 5 a similar distribution of hoop stress is shown for analuminium strip being coiled on a spool having the shape described abovewith reference to FIG. 3. Here it can be seen that the coiling tensionis carried by the middle 500 mm of the strip width throughout coilingand no tension pockets will be formed in the inner position after themandrel is removed from the spool. Thus, using a spool shape that isconvex with a crown across its centre region, a strip with improvedflatness can be achieved. Even a small variation in the outer diameterof the spool at its central region can produce a dramatic effect to thecoil stress.

Although it may be difficult to construct a spool having the shapedescribed in FIG. 3, shapes capable of achieving similar improvements insheet flatness can be easily constructed. For example, an approximatelycylindrical deformable spool may be used in conjunction with a mandrelthat varies in diameter between the centre of the spool and the spooledges. If the mandrel has a positive crown the spool deforms to asimilar crown. Ideally, the mandrel is constructed so that the spool isnot in contact with the mandrel either side of the central crown region.

However, the preferred spool structure utilises a length of strip tocreate a raised crown for the centre region of a plain cylindricalspool. For example, a conventional cylindrical spool, having uniformdiameter, is converted by means of a short length of metallic (e.g.aluminium) strip having a gauge of approximately 0.28 mm gauge and awidth of around 525 mm which is wound with one or more turns around thecentre region 2 of the spool to form a sleeve about the centre region ofthe spool. The aluminium strip to be coiled is then wound around theoutside of the converted spool in the usual manner. It will, of course,be appreciated that the sleeve need not be made from a metallic materialand may instead be of natural fibre, plastic or other durable material.Also, as the sleeve is a separate part of the spool it can easily beconstructed to the desired gauge and width. FIG. 6 shows thedistribution of hoop stress for the same three positions using theconverted spool described above and as can be seen the effect of usingthe converted spool is similar to that of FIG. 5. In particular quarterpockets on the inner laps of the strip are avoided. FIG. 6 was producedon the basis of a spool having a rectangular crown 460 mm wide. Therectangular crown concentrates the hoop stress of the inner laps, forexample after 5 mm build-up, into the same width as the stress in thesubsequent laps is concentrated by the coil crown. Thus, the effect ofthe increased spool diameter in the central portion of the strip is toreduce the width-wise range of hoop stress in the inner laps after themandrel has been removed. This can be seen by comparing the hoop stresscurves for the first position in FIG. 4 with the corresponding curve inFIG. 6. The difference comes about because the region of increaseddiameter supports the central part of the coil, leaving the outerregions unsupported and thus with low absolute hoop stress.

In a further alternative embodiment, illustrated in FIG. 7, aconventional plain cylindrical spool (not shown) may be used for coilingan aluminium strip 10. In order to provide the crown at the centreregion of the spool, the leading end of the strip is shaped to form atongue 11 having a width less than that of the strip 10. With theleading edge 12 of the tongue 11 centred on the spool, the first one ormore laps of the strip build up to form a crown at the centre region ofthe spool. Thereafter, the strip 10 becomes full width and coiling ofthe strip continues in the usual manner. In this way the leading end ofthe strip itself is used to create the convex surface of the spool toensure that the tensile stress is applied to the centre region of theinnermost laps of the strip at its full width. FIG. 7 shows threepossible shapes for tongue 11. In FIG. 7A, the tongue is rectangular inshape, with a substantial step change to full width (although inpractice corners would preferably be rounded to reduce stress). In FIGS.7B and 7C, a gradual transition from the leading edge 12 to full widthis used, thus reducing the likelihood of snatching of the exposedcorners as the strip passes through the processing machinery. Althoughconcave curves are shown in FIGS. 7B and 7C, straight sides could alsobe used, the best shape for the circumstances being determined byexperiment.

The length l of the tongue should be at least equal to a single turnaround the circumference of the spool; however, if this does not givesufficient thickness a longer tongue can be used, preferably of lengthequal to a multiple of the circumferential length of the spool, sinceother than a multiple would lead to unbalanced forces during coiling.

In a still further alternative, illustrated in FIG. 8, a conventionalplain cylindrical spool (not shown) is used, and the strip 10 adapted byattaching to one face, at the leading end, a sheet 13 of thin material.This material may, for example, be aluminium which is attached byadhesive. It will be seen that, as the strip 10 is coiled around thespool, the thickness of sheet 13 acts to increase the effective diameterof the spool in the central section of the width of the strip 10, thusgiving the same effect as described above. One or more further sheets(not shown) may be attached on top of sheet 13 to increase thethickness, as required, and these extra sheets may be attached to theopposite surface of strip 10. The “extra” sheet or sheets thus appliedneed not necessarily be the same size as sheet 13, but could be smallerto provide a stepped edge or edges to sheet 13.

The length of sheet 13 in the longitudinal direction of the strip 10will be at least equal to the circumferential length of the spool andpossibly a multiple thereof, as discussed above with reference to thetongue 11 of FIG. 7.

In FIG. 9 the creep strain across the width of the first position 24hours after coiling is illustrated for a conventional right cylindricalspool, a spool having a convex (positive) crown, and a spool having edgesleeves. In FIG. 9, creep strain is given in i-units which are definedasε_(r)·10⁵where ε_(r) is the relative strain, given by:ε_(r) =ΔL/L _(a)where

-   -   ΔL=change in length    -   L_(a)=average of original lengths of all positions across the        width of the strip

As can be seen in FIG. 9, for the conventional spool the strain extendsover the middle 800 mm of the strip width so that the strip at theinnermost superlap is likely to exhibit wavy edge off-flatness. For astrip coiled on a convex spool the strain extends over only the middle500 mm of width and will exhibit less wavy edge off flatness. The spoolwith the edge sleeves produces massive differences in strain between thecentre and the edge and consequently a large off flatness. This lattercorresponds approximately to the prior art spool of JP 11 17 94 22.

In FIG. 10 the flatness change over the entire length of the aluminiumstrip in terms of creep strain (in i units) is illustrated and may becompared with FIG. 1 for a conventional spool. Most notably, for theinner laps the positive strain towards the edges of the strip in FIG. 1is missing from FIG. 10. Also the magnitude of any wavy edge effects isgreatly reduced in FIG. 10. FIG. 10 thus illustrates that theoff-flatness effects likely to be found using conventional coilingmethods can be avoided or at least reduced using the contoured spool andthe coiling method described above.

The positive contours of the spool may also be achieved by weakening theaxial ends of the spool. For example, slits may be cut into the ends ofthe spool up to a distance of approximately ¼ the width of the spoolwhich would cause the ends to collapse under the compressive load of thecoil (for example when the ends are not supported by the mandrel or whenthe support from the mandrel is withdrawn) to form a central convexcrown. Here too the beneficial shape is adopted by the spool only aftera few laps of the aluminium strip. In a further alternative, the centralregion of the spool may be constructed of a different material to thatof the edge regions with the material of the central region being morerigid so that as the strip material is coiled onto the spool the edgeregions produce a greater deflection in response to the compressive loadof the laps than the central region.

The above description has focused on utilising a convex spool to reducethe off-flatness effects of a coiled aluminium strip. It is alsopossible to control off-flatness effects through controlling andadjusting the tension of the strip as it is being coiled. To reduceoff-flatness effects the tension applied to the strip must be higher,for example up to 30 MPa, for the initial laps of the coil and then bereduced to a lower tension for the outer laps of the coil. Thisreduction in tension can extend over up to half the entire length of thestrip. However, it is preferable if the reduction in tension is limitedto the first third of the entire strip length.

The earlier model predictions for a convex spool were all generatedassuming that the maximum coiling tension for the initial laps was abouttwice that of the outer laps the reduction being effected over about thefirst 25 mm of build up of the coil (referred to as the conventionalpractice). In FIGS. 11 and 12 the effect of coiling tension on theflatness of aluminium strip coiled onto a convex spool is illustrated.In FIG. 11 creep strain along the centre line of the strip, immediatelyafter coiling, is plotted for an aluminium strip coiled onto aconventional plain spool using conventional practice; onto a convexspool using conventional practice; onto a convex spool using an initialcoiling tension of 10 MPa; and onto a convex spool using an initialcoiling tension of 15 MPa. In the last two cases, the coiling tensionwas decreased exponentially to about half the original value during thefirst 15 mm build up of the coil. As the coil continues to build up, itcan be advantageous to decrease the tension still further to a levelthat does not cause significant creep to occur e.g. to around 10 to 50%of the starting tension. It can be clearly seen from FIG. 11 that theuse of a convex spool in combination with a much higher initial coilingtension greatly increases the creep strain in the strip for the innerlaps of the coil and indeed that the larger the initial tension, thelarger the long middle strain in the inner laps during coiling. In FIG.12, which provides the same examples for comparison but for creep strain24 hours after coiling, it can be seen that the larger the initialcoiling tension the smaller the compressive strain in the inner lapsafter 24 hours. From FIG. 12 for an initial coiling tension of 15 MPa,the strip is flat for the laps very close to the spool and then a wavyedge builds up at around 25 mm.

Whilst details are given of different structures of spools and differentmethods of adjusting coil tension for enabling the stress in the innerlaps to be adjusted, these are only examples and the spirit and scope ofthe present invention is not restricted to the particular examples givenabove.

EXAMPLE

AA1050 sheet cold rolled to a thickness of 0.28 mm and width of 1050 mmwith a positive crown profile was wound into coils 1750 mm in diameterusing the conventional practice. Four coils were made one on each of thefollowing spools:

-   -   1) Cylindrical spool (comparative example)    -   2) Cylindrical spool as in (1) but with eight equally spaced        slits in each end of the spool extending to the edge of the        central 500 mm region.    -   3) As in (1) but with a single lap of 0.15 mm thick 500 mm wide        aluminium strip wound round the centre of the spool    -   4) As in (3) but with a strip 0.3 mm thick

24 hours after coiling the coils were unwound and flatness samples 4 mlong were taken at intervals along the entire length of the sheet.Samples were taken closer together towards the spool end of the coilthan at the start. Flatness was measured by placing the samples on aflat steel table and measuring the levels of any off-flatness,represented as strain in i-units, by means of displacement transducers.The results are plotted in FIGS. 13 to 16 respectively showing contoursof levels of off-flatness for various positions in the coil. The samecontour steps, of 0.25 i-units, have been used for all graphs. From thefigures it will be seen that the crowned spools reduced the level ofoff-flatness by a factor of about 2.5. This is a significantimprovement.

1. A system for coiling of aluminum strip material having a coilassembly comprising a mandrel, a spool removably mounted on the mandreland an aluminum strip material having a positive crown, the coilassembly having a supporting surface for coiling the strip material, thesupporting surface providing a support profile in which that part of thesupporting surface which supports the crown has a greater diameter thanremaining parts of the supporting surface during coiling of inner lapsof the strip material, wherein the spool has a length at least equal tothe width of the strip material.
 2. a system as claimed in claim 1,wherein the support profile of the supporting surface is provided byadaptation of the spool.
 3. A system as claimed in claim 2 wherein thespool is cylindrical, of substantially uniform diameter, and has slitsextending from one or both ends of the spool.
 4. A system as claimed inclaim 3, wherein the slits extend approximately one quarter of thelength of the spool.
 5. A system as claimed in claim 2, wherein thespool has an outer diameter at that part of the spool which supports thecrown that is greater than the outer diameter of the spool at one orboth opposing end regions of the spool.
 6. A system as claimed in claim5, wherein the spool is contoured to have an outwardly projecting crownover said part of the spool.
 7. A system as claimed in claim 6, whereinthe outwardly projecting crown is rectangular.
 8. A system as claimed inclaim 2, wherein that part of the spool which supports the crown isformed of a material having greater rigidity than that of a materialforming one or both of the opposing end regions of the spool.
 9. Asystem as claimed in claim 1, further including at least one tensioningroll and a tension control device to control tension of the stripmaterial as it is coiled from a first higher tension to a second lowertension.
 10. A system as claimed in claim 1, wherein the support profileof the supporting surface is provided by means separate from the spool.11. A system as claimed in claim 10, wherein the spool is cylindrical inshape.
 12. A system as claimed in claim 11, wherein the means separatefrom the spool that provides a support profile of the supporting surfaceis an outer sleeve mounted about that part of the spool which supportsthe crown, the outer sleeve having a width less than the width of thestrip material.
 13. A system as claimed in claim 11, wherein the meansseparate from the spool is a length of material which is wound one ormore times around the spool prior to coiling, the length of materialhaving a width narrower than that of the strip material.
 14. A system asclaimed in claim 10, wherein the means separate from the spool thatprovides the support profile of the supporting surface is the stripmaterial shaped to have, when wound, the greater diameter at the partthat supports the crown.
 15. A system as claimed in claim 14, wherein aleading end of the strip material is formed as a tongue having a widthnarrower than the width of the strip material.
 16. A system as claimedin claim 15, wherein the length of the tongue, in the longitudinaldirection of strip material, Is approximately equal to n times the outercircumference of the spool, wherein n is an integer greater than zero.17. A system as claimed in claim 10, wherein the means separate from thespool is a sheet of material is attached to a surface of a leading endof the strip material, the sheet having a width narrower than that ofthe strip material.
 18. A system as claimed in claim 17, wherein thesheet of material has a length, in the longitudinal direction of thestrip material, which is approximately equal to n times the outercircumference of the spool, where n is an integer greater than zero. 19.A system as claimed In claim 18, wherein the sheet is made of aluminum.